1. Field of the Invention
The present invention relates to a resonance testing device and method for use in hard disk drive actuators. More particularly, the present invention relates to a resonance testing device using an accelerometer to determine the range of horizontal motion of a hard disk drive actuator during operational motion.
2. Description of the Related Art
Hard disk drive (HDD) storage devices currently store data on a magnetic hard disk drive by magnetically reading data to and writing data from a disk via a read/write head (r/w head) fixed to an actuator arm.
Alternatively, newer drives, called MR drives, write using magnetic physics which changes the magnetic field on the media. MR drives read by using the variation of electrical resistance of the r/w head. In this way, the electrical resistance of the MR r/w head is changed if it is located near the written data, i.e., variation in the magnetic field, on the media. As a result, by monitoring the variation of electrical resistance of the r/w head, it is possible to read the information written on the media. With a thin film head, the HDD measures the electrical current flow generated when the r/w head is located on the different magnetic field.
However, regardless of the type of drive, during operation the disk rotates about its axis, while the actuator arm moves the r/w head across the disk. The actuator arm moves the r/w head to different areas of the disk to allow the r/w head to read data from and write data to the disk. The disk itself is divided into a number of concentric tracks each having the same width. These tracks are in turn divided into a number of sectors. In seeking out a particular track, the actuator head moves in a radial direction from its current location to the location of the track in which the data sector it is seeking is located.
For the r/w head to operate properly, it should perform its function at a distance in the tens of microns above the surface of the hard disk. If the distance between the r/w head and the disk gets too small, if impurities form on the surface of the disk, or if the head moves too much in a vertical direction towards the disk, the r/w head can impact on the surface of the disk, causing damage to the head and the disk. This undesired collision is called a head crash.
In addition, for the r/w head to operate properly, it must also be moved to the desired track and sector of the disk within a narrow horizontal range as well. Too much horizontal displacement can cause the r/w head to be improperly aligned over the desired track and sector. A horizontal displacement of as little as 8 microns can cause the disk drive to fail to operate properly.
An inherent limitation in the read/write process is the fact that the actuator arm and the r/w head will oscillate slightly in a horizontal direction as they move back and forth. Since the r/w head must stay very small margin of horizontal movement when seeking a particular track, the oscillation must be kept to within a very small tolerance for the HDD to operate properly. Too much oscillation will result in the very real possibility of an improper alignment of the r/w head during a point of maximum oscillation, meaning a failure to read or write data properly.
FIG. 1 is a graph for showing the natural frequencies of an actuator arm structure. As shown in FIG. 1, the actuator arm has a gain of the actuator arm varies depending upon the frequency of its movement. Under ideal conditions, the gain is kept close to 0 dB so that the gain will be even and the actuator arm will move as directed. As FIG. 1 shows, however, the gain of the system rises dramatically near the natural, or modal, frequencies f.sub.M1 and f.sub.M2 (although only two natural frequencies are shown, there may be others as well). It is important to identify what these frequencies are so that they can be avoided in the operation of the actuator system.
One way that the graph of natural frequencies can be obtained is by performing a modal analysis using parameters for a tortional and bending mode. This will give an acceptable graph for determining a reasonable operating frequency for the tested actuator, but requires measurements of the actuator resonance.
Thus, it is very helpful to measure the actuator resonance for HDDs prior to sale or use of the HDD devices. The actuator resonance serves as a measurement of the horizontal motion of the actuator arm and r/w head assembly, and will help determine the likelihood that the head will be able to move to the desired track of the disk for the tested actuator. An indication of actuator resonance will help decrease failures in manufactured HDD devices, and can serve to increase the yield of an HDD manufacturing process. If the actuator resonance is measured prior to the operation of the HDD device, the danger of improper head alignment can be accurately identified and avoided. This can serve to enhance the reputation of the company using this testing scheme and thereby increase its financial success. As a result, although an effective actuator resonance test is not essential in and HDD manufacturing process, it is desirable.
In the past, the actuator resonance has been measured through the use of a laser-based testing system, comprising a laser Doppler vibrometer, a digital signal analyzer, a precise x-y-z fixture, and a high-fidelity power amplifier. An example of this conventional method is shown in "Drive Level Slider-Suspension Vibration Analysis And its Application to a Ramp-Load Magnetic Disk Drive," by Ta-Chang Fu, et al., IEEE Transactions on Magnetics, Vol. 31, No. 6, (November 1995), the contents of which are hereby incorporated by reference.
One disadvantage of this kind of a laser-based system is its high cost. Typically such a laser-based resonance tester costs $80,000 or more. This precludes many small parts suppliers from being able to afford to engage in actuator resonance testing.
In addition, to use the laser-based actuator resonance tester, it is necessary to have an operator with a certain level of knowledge about laser characteristics and laser operation. Such an operator must have sufficient expertise in the field of lasers to get a stable beam focus for the laser used in the system. This serves as a further barrier to the use of laser-based actuator resonance testers. It also serves to increase costs when laser-based system are indeed used. It also adds to the time and complexity of the testing process, since beam focusing is neither simple nor quick.
A laser based testing scheme also requires that a hole be drilled in each drive unit tested to allow the laser beam to shine on the relevant components. This hole must be drilled with a certain amount of precision, which further increases the complexity and cost of testing. Furthermore, the drilled hole serves to ruin the HDD device for any future use, again increasing the cost of testing.
A further disadvantage of conventional laser-based actuator resonance testers is that they require that the HDD head-disk assembly be fully constructed before it can be tested. If the HDD device is not fully assembled prior to testing, the weight balances will not be correct and the measured actuator resonance of the device will be inaccurate. For the testing to be accurate, the same boundary conditions must exist during testing as would exist in the final HDD device. Since both ends of the pivot in a manufactured HDD device are fixed, i.e., connected to the manufactured HDD device, they must both be fixed in the testing device. The way this is achieved in conventional testing methods is to fully assemble the HDD device prior to testing. Not only does this increase the cost and time required for such a test, but it precludes the actuator vendors from being able to make the tests prior to final assembly, since they do not have access to the assembled devices.
It is therefore desirable to provide an affordable actuator resonance tester for a disk drive that can be operated without a great deal of specialized knowledge and prior to final assembly of the disk drive.